A MODIFIED CATALYST FOR OPERATING ELECTROCHEMICAL CARBON DIOXIDE REDUCTION IN A NON-ALKALI ACIDIC MEDIUM AND RELATED TECHNIQUES

Description

TECHNICAL FIELD

The present invention generally relates to electrochemical carbon dioxide reduction (CO₂R), and more particularly to a CO₂R catalyst for operating electrochemical CO₂R into carbon products in a non-alkali acidic medium, and related cathode, system, process, and method.

BACKGROUND

Electrochemical CO₂R offers a route to produce fuels and chemicals with low carbon intensity. CO₂R towards multi-carbon (C₂₊) products has reached Faradaic Efficiencies (FEs) of 70%-80% at relevant current densities (>100 mA cm⁻²) in CO₂R-favorable alkaline and neutral reaction environments. However, at these conditions, the loss of reactant CO₂ to carbonate species limits a single pass CO₂ conversion efficiency (SPC) (<5%) and thus leads to a significant extra energy cost to regenerate CO₂ from the electrolyte.

Acidic electrolytes can eliminate carbonate formation, enabling high SPC (70%-80%); but CO₂R kinetics become outcompeted by the hydrogen evolution reaction (HER) at low pH. Adding alkali cations to the electrolyte can help steer the reaction to CO₂R. However, alkali cations which are essential for CO₂R have low solubility limits in acidic conditions, leading to salt accumulation on the catalyst and gas diffusion cathode that limits the lifetime of the cell (e.g., limiting operating stability to <15 hours).

There is thus a need for improved techniques that overcome at least some of the drawbacks of what is known in acidic CO₂R, including instability, salt formation and HER competition.

SUMMARY

Implementations of a CO₂R catalyst including a fixed cationic functional group respond to the above need by lowering a proton diffusion rate and thereby providing a local alkaline pH at the surface of the to improve CO₂ adsorption and C—C coupling. More particularly, in one aspect, there is provided a carbon dioxide reduction (CO₂R) catalyst for sustaining the electroreduction of carbon dioxide (CO₂) into carbon products in a non-alkali acidic medium, the CO₂R catalyst comprising:

• • a catalytic layer being electrically conductive and comprising a metal catalyst favouring CO₂R reactions, and
  • an ionic layer being ionically conductive and deposited onto the catalytic layer, the ionic layer comprising an ionomer, with the ionomer comprising a cationic functional group being covalently bonded to a polymeric backbone of the ionomer and adsorbed on the surface of the catalytic layer.

The CO₂R catalyst of the present disclosure allows direct contact between the electrolyte necessary for the electrochemical reduction of CO₂ into carbon products and the cationic functional group of the ionomer. The electrolyte further allows high ionic conductivity and enhanced triple-phase boundaries. The ionomer not only provides functional groups to allow intermolecular interaction of water with surface adsorbed CO but can also enhance CO₂ adsorption at the catalyst surface, and provides hydrophobic functionality to slow down proton transport in strong acidic media. The CO₂R catalyst helps to solve the carbonate (salt) formation problem that is specifically present in acidic flow systems and prolongs the stability from tens of hours to hundreds of hours while providing conditions favouring CO₂RR over HER in acidic media.

For example, the catalytic layer can have a thickness ranging between 100 nm and 1000 nm as determined by scanning electron microscope cross-section measurements; preferably ranging from 110 to 800 nm; more preferably from 120 to 600 nm; even more preferably from 130 to 500 nm; most preferably from 140 to 400 nm; and even most preferably from 150 to 300 nm; or from 160 to 250 nm.

For example, the metal catalyst is copper, nickel, cobalt, tin, bismuth, lead, indium, ruthenium, palladium, zinc, silver, gold, any alloys thereof or any combinations thereof. With preference, the metal catalyst is or comprises copper.

For example, the cationic functional group can be or can comprise an imidazolium moiety. For example, the cationic functional group can be or can comprise a benzimidazolium moiety. For example, the cationic functional group can be or can comprise trimethylammonium, triethylammonium, tributylammonium, tripropylammonium, imidazole, 2-methyl-imidazole, 1,3-dimethyl-imidazole, 1-ethyl-imidazole, 1,3-diethyl-imidazole, 9-carbazole, N-ethyl-carbazole, pyridine, or any mixtures thereof.

For example, the ionic layer can have a thickness ranging between 10 and 50 μm as determined by scanning electron microscope cross-section measurements; preferably between 12 to 45 μm; more preferably between 15 to 40 μm; and even more preferably from 18 to 30 μm.

Optionally, the ionic layer can have an ionomer loading ranging between 1 and 5 mg cm⁻²; preferably between 1.2 to 4.8 mg cm⁻²; more preferably between 1.5 to 4.5 mg cm⁻²; even preferably between 1.7 to 4.2 mg cm⁻²; most preferably between 2.0 to 4.0 mg cm⁻²; even most preferably between 2.2 to 3.8 mg cm⁻²; or between 2.5 to 3.5 mg cm⁻².

Optionally, the ionic layer can have an ion exchange capacity between 0.5 and 2.6 meq·g⁻¹ in accordance with a loading of the cationic functional group, the ion exchange capacity being determined by the number of moles of exchanged ions to the dry weight of ionomer, for example OH⁻, Cl⁻, and/or I⁻ ions. Further optionally, the ion exchange capacity can be between 0.5 and 1.4 meq·g⁻¹; preferably between 0.5 and 1.2 meq·g⁻¹; more preferably between 0.5 and 1.0 meq·g⁻¹; even more preferably between 0.5 and 0.8 meq·g⁻¹; and most preferably between 0.5 and 0.6 meq·g⁻¹. Further optionally, the ion exchange capacity can be between 1.0 and 2.3 meq·g⁻¹; preferably between 1.1 and 2.1 meq·g⁻¹; more preferably between 1.2 and 2.0 meq·g⁻¹; even more preferably between 1.3 and 1.9 meq·g⁻¹; and most preferably between 1.4 and 1.7 meq·g⁻¹. Further optionally, the ion exchange capacity can be between 1.8 and 2.6 meq·g⁻¹; preferably between 2.0 and 2.6 meq·g⁻¹; more preferably between 2.1 and 2.6 meq·g⁻¹; even more preferably between 2.3 and 2.6 meq·g⁻¹.

For example, the ionomer can be an alkaline ionomer. Optionally, the ionomer can be Aemion™, PiperION, or Sustainion®.

In another aspect, there is provided a modified cathode for operating electroreduction of carbon dioxide (CO₂) into carbon products in a non-alkali acidic medium, the modified cathode comprising a gas diffusion layer, and the CO₂R catalyst as defined herein, the catalytic layer of the CO₂R catalyst being deposited onto the gas diffusion layer.

In some implementations, the modified cathode further comprises an additional physical barrier layer deposited onto the ionic layer of the CO₂R catalyst to enhance stability of the CO₂R catalyst in the non-alkali acidic medium. For example, the additional physical barrier layer can be a carbon-containing layer. Optionally, the carbon-containing layer can include carbon nanotubes, graphite, or a combination thereof.

For example, the gas diffusion layer can be porous and has a pore size between 0.3 and 1 μm as determined by scanning electron microscope (SEM); preferably from 300 to 900 nm; more preferably from 350 to 800 nm; even more preferably from 400 to 600 nm.

For example, the gas diffusion layer can be porous and has a porosity between 50% and 90% as determined by porosimeter.

In another aspect, there is provided a system for operating electroreduction of carbon dioxide (CO₂) into carbon products in a non-alkali acidic electrolyte, the system comprising:

• • a cathodic compartment comprising:
    • a reactant inlet configured to be supplied with a stream of gaseous CO₂,
    • a modified cathode as defined herein converting CO₂ into carbon products according to CO₂R reactions,
    • a product outlet to release a gas-liquid mixture comprising the carbon products;
  • an anodic compartment comprising:
    • an anodic inlet configured to be supplied with a non-alkali acidic anolyte;
    • an anode converting H₂O into O₂,
    • an anodic outlet configured to release a mixture of O₂ and used non-alkali acidic electrolyte; and
  • a proton exchange membrane separating the cathodic compartment and the anodic compartment.

For example, the system can be a flow cell and the cathodic compartment further comprises a catholyte inlet configured to receive the non-alkali acidic electrolyte as a catholyte, and a catholyte outlet to release used catholyte. Optionally, the flow cell can be a slim flow cell comprising a catholyte flow field having a thickness between 0.4 and 2 mm.

For example, the non-alkali acidic electrolyte can have a pH of at most 7; preferably below or equal to 6, more preferably below or equal to 5, even more preferably below or equal to 4, most preferably below or equal to 3, even most preferably below or equal to 2, or below or equal to 1.

Optionally, the non-alkali acidic electrolyte can be a solution of hydrochloric acid, sulfuric acid, hydrobromic acid, hydriodic acid, perchloric acid, chloric acid and any mixture thereof having a concentration between 0.01 and 0.5 M; preferably a solution of sulphuric acid, phosphoric acid or perchloric acid having a concentration between 0.01 and 0.5 M. For example, non-alkali acidic electrolyte comprises a solution of sulphuric acid.

For example, the non-alkali acidic electrolyte can be a solution of hydrochloric acid, sulfuric acid, hydrobromic acid, hydriodic acid, perchloric acid, chloric acid and any mixture thereof having a concentration between 0.01 and 1 M; preferably from 0.01 to 0.7 M; more preferably from 0.01 and 0.5 M; even more preferably from 0.05 to 0.45 M; most preferably from 0.1 to 0.4 M; and even most preferably from 0.1 to 0.3 M.

For example, the proton exchange membrane can be or can comprise perfluoro(2-(2-sulfonylethoxy)propyl vinyl ether)-tetrafluoroethylene copolymer, or tetrafluoroethylene-perfluoro(3-oxa-4-pentenesulfonic acid) copolymer.

In another aspect, there is provided a process for electrochemically reducing CO₂ into carbon products, wherein the process comprises the following steps:

• • (a) providing a system as defined herein,
  • (b) supplying the gas stream of CO₂ to the system,
  • (c) supplying the non-alkali acidic electrolyte to the system, and
  • (d) recovering the gas-liquid mixture comprising the carbon products.

For example, step (b) can be performed at a gas flow rate between 0.2 and 150 sccm, optionally between 0.2 and 100 sccm or between 0.2 and 50 sccm.

For example, the process includes applying a full-cell potential sufficient to achieve a current density that can be between 10 and 500 mA·cm⁻², as applied by electrochemical potentiostat stations. Optionally, the full-cell potential can be between 2.75 and 4 V, further optionally between 2.75 and 3.4 V.

For example, step (d) can be performed for a duration of at least 100 hours.

For example, step (d) can be performed with a C₂+ Faradaic Efficiency (FE) of at least 80% and an H₂ FE to at most 10%.

In yet another aspect, there is provided a method for manufacturing a modified cathode configured for operating electroreduction of carbon dioxide (CO₂) into carbon products in a non-alkali acidic medium, wherein the method comprises the following steps:

• • (a) providing a gas diffusion layer,
  • (b) depositing a metal catalyst favouring CO₂R reactions onto the gas diffusion layer provided at step (a) to form an electrically conducting catalytic layer, and
  • (c) depositing an ionomer onto the electrically conducting catalytic layer formed at step (b) to form an ionically conducting layer, the ionomer comprising a cationic functional group bonded to a polymer backbone of the ionomer.

For example, step (b) can comprise sputtering the metal catalyst onto the gas diffusion layer.

Optionally, the method can comprise controlling a deposition rate of the metal catalyst in accordance with a given thickness of the electrically conducting catalytic layer. For example, the deposition rate can be between 0.7 and 2 Å s⁻¹.

For example, step (c) can comprise spraying a solution comprising the ionomer. Optionally, the method can comprise controlling a spray loading of the solution in accordance with a given thickness of the ionically conductive layer. For example, the spray loading of the ionomer can be between 1 and 5 mg·cm⁻².

In some implementations, the method can further comprise the step of preparing the solution comprising the cationic functional group. The step of preparing the solution comprising the ionomer can include dissolving an ionomer powder in a solvent. Optionally, the solution of ionomer can have an ionomer concentration between 0.5 wt. % and 5 wt. % based on the total weight of the solution of ionomer; preferably from 0.5 to 4 wt. %; more preferably from 0.6 to 3 wt. %; even more preferably from 0.7 to 2 wt. % and most preferably from 0.8 to 1.5 wt. %

In some implementations, the method can further comprise the step of depositing a physical barrier layer onto the ionically conductive layer formed at step (c). For example, the step of depositing the physical barrier layer can include spray coating a carbon-containing ink onto the ionically conductive layer formed at step (c). For example, the carbon-containing ink can be an ionomer solution comprising carbon nanoparticles, graphite or a combination thereof. For example, the ionomer of the ionomer solution can be Nafion™ or Aquivion®.

While the present techniques will be described in conjunction with example embodiments and implementations, it will be understood that it is not intended to limit the scope of the invention to such embodiments and implementations. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included as defined by the present description. The objects, advantages and other features of the present techniques will become more apparent and be better understood upon reading of the following non-restrictive description, given with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Implementations of the CO₂R catalyst and related performance in a non-alkali acidic medium are represented in and will be further understood in connection with the following figures.

FIG. 1 includes two comparative schematic representations of (a) a metal cation activated CO₂R catalyst, and (b) a fixed cation functional groups activated CO₂R catalyst.

FIG. 2 includes (a) a schematic representation of a carbon protected CO₂R catalyst-(b) a graph of DFT calculations when replacing metal alkali cations by fixed-cation functional groups in acidic CO₂R reactors showing calculated Gibbs free energy of CO₂ adsorption and energy barrier of C—C coupling—(c) a schematic representation of corresponding optimized geometries for CO₂ adsorption and transition state of C—C coupling.

FIG. 3 includes two graphs showing FE toward gas products on bare Cu catalyst in H₂SO₄ electrolyte with no metal cation addition with CO₂R being carried out at current density of (a) 50 mA cm⁻² and (b) 100 mA cm⁻².

FIG. 4 is a graph showing FE toward gas products on bare Cu catalyst when the system is operated at a current density of 100 mA cm⁻² in 0.01 M H₂SO₄+KCl solution with a KCl concentration varying from 1 to 3 M.

FIG. 5 is a photograph of a conventional cathode surface showing solid salt formation after 1 h of operation with the experiment being carried out on bare Cu in 0.01 M H₂SO₄+3 M KCl electrolyte.

FIG. 6 is a graph showing a catalytic performance of a CO₂R catalyst with different fixed-cationic functional group loadings upon operating in 0.2 M H₂SO₄ electrolyte at a current density of 100 mA cm⁻².

FIG. 7 includes five graphs relating to operation of bare copper (Cu) catalyst and the present CO₂R catalyst (CG-low Cu) in a flow cell and in an acidic medium, i.e., with 0.2 M H₂SO₄ as catholyte with a pH of 0.4: graph (a) showing selectivity of bare Cu at a current density of 100 mA cm⁻² and CG-low Cu at current densities of 50 and 100 mA cm⁻², with values being means, and error bars indicating SD (n=3 replicates)—graph (b) showing CV curves of bare Cu and CG-low Cu in 0.2 M H₂SO₄ with Ar and bare Cu in 0.2 M H₂SO₄+3 M KCl with Ar, with scan rate being 100 mV s⁻¹ and CV (cyclic voltammetry)—graph (c) showing selectivity of bare Cu and CG-low Cu in 0.2 M H₂SO₄ solution with and without K⁺ at a current density of 100 mA cm-², with values being means, and error bars indicating SD (n=3 replicates)—graph (d) showing FE and potential degradation of CG-low Cu at a constant current density of 100 mA cm⁻² in 0.2 M H₂SO₄ with 3 M K⁺ as catholyte during 30 minutes, with values being means, and error bars indicating SD (n=3 replicates) and formation of H₂ bubbles affecting the reference electrode, and gradually increasing noise in the collected potential signals—graph © showing ex situ and in situ Raman spectroscopies of CO adsorption on CG-low Cu, with the in situ Raman spectroscopies being obtained at a constant current density of 20 mA cm⁻² in a 0.05 M H₂SO₄.

FIG. 8 is a graph showing catalytic performance of the present CO₂R catalyst (CG-low Cu) in 0.2 M H₂SO₄+KCl electrolyte with KCl concentration ranging from 0 to 3 M, and the operating current density being 100 mA cm⁻².

FIG. 9 is a graph showing CV characterizations of present CO₂R catalyst (CG-low Cu) in acid electrolytes, with the CV curves being recorded in the condition of 0.2 M H₂SO₄ with (dash line) and without K⁺ (solid line), respectively.

FIG. 10 includes four graphs relating to double layer capacitance measurements. Graph (a) shows CV curves of bare Cu in 0.2 M H₂SO₄+3 M KCl solution, graph (b) shows CV curves of CG-low Cu in 0.2 M H₂SO₄, and graph (c) shows CV curves of CG-low Cu in 0.2 M H₂SO₄+3 M KCl recorded at different sweep rates with Ar. The insets show a plot of the current density recorded at −0.17 V with respect to the sweep rates. The slope of the dotted line was used to calculate the double layer capacitance. Graph (d) shows an electric double layer (EDL) capacitance comparison of CG-low Cu in 0.2 M H₂SO₄ and bare Cu and CG-low Cu in 0.2 M H₂SO₄ with 3 M K⁺ on the catalytic performance of CG with different functional group density at a current density of 100 mA cm⁻².

FIG. 11 includes six graphs relating to performance of the fixed-cation catalysts in 0.2 M H₂SO₄ electrolyte. Graph (a) shows an Ex situ Raman spectroscopy of CG-low, CG-medium and CG-high. The characteristic vibration of benzyl group is from 900 cm⁻¹ to 1100 cm⁻¹. Graph (b) shows an EDL capacitance comparison of CG-low, CG-medium and CG-high in 0.2 M H₂SO₄ solution. Graph (c) shows In situ Raman spectroscopies of CO adsorption on CG-low, CG-medium and CG-high Cu. The in situ Raman spectroscopies were obtained at a constant current density of 20 mA cm⁻² in a 0.05 M H₂SO₄. Graph (d) shows CV curves of bare CG-low, CG-medium and CG-high in 0.2 M H₂SO₄ with Ar. Scan rate was 100 mV s⁻¹. Gra©(e) shows a selectivity comparison of CG-low, CG-medium and CG-high in 0.2 M H₂SO₄ solution at a current density of 100 mA cm⁻². Values are means, and error bars indicate SD (n=3 replicates). Graph (f) shows a selectivity comparison of CG-medium Cu at different operating current densities. The tests were carried out in 0.2 M H₂SO₄ with a CO₂ flow rate of 50 sccm. Values are means, and error bars indicate SD (n=3 replicates).

FIG. 12 is a graph showing catalytic performance of the present CO₂R catalyst (CG-medium Cu) operating in different H₂SO₄ concentration of the catholyte, with operation in the catholyte at 0.2 M H₂SO₄ showing a smaller reduction potential compared to 0.05 and 0.1 M H₂SO₄ catholyte due to the lower ohmic resistance.

FIG. 13 includes three graphs providing a Raman characterization for a given cationic functional group (benzyl group) at 900 to 1100 cm⁻² based on Ex situ and in situ Raman spectroscopy of CG-low Cu (graph (a)), CG-medium Cu (graph (b)), and CG-high Cu (graph (c)). The in situ Raman spectroscopies were obtained in a 0.05 M H₂SO₄ and 0.05 M H₂SO₄+3M KCl solution, respectively. All in situ data were collected at a constant current density of 20 mA cm⁻².

FIG. 14 includes three graphs providing electrochemical characterization of the present CO₂R catalyst at different concentrations of cationic functional group (CG-low, medium and high Cu). Graph (a) show CV curves for CG-low, graph (b) for CG-medium, and graph (c) for CG-high in 0.2 M H₂SO₄ electrolyte with (dash) and without (solid) 3M KCl addition. The scan rate was 100 mA cm⁻². All data were collected under the Ar condition.

FIG. 15 is a graph showing performance of the present CO₂R catalyst (CG-medium Cu and CG-high Cu) in 0.2 M H₂SO₄ with excess cation, with the experiments being operated in a 0.2 M H₂SO₄+3 M KCl electrolyte at a current density of 100 mA cm⁻².

FIG. 16 includes two graphs relating to performance degradation during the initial 30 min of CO₂R, with graph (a) showing FE degradation of CG-medium Cu and graph (b) showing FE degradation of CG-high Cu at a constant current density of 100 mA cm⁻², and 0.2 M H₂SO₄ with 3 M K⁺ being used as the catholyte during 30 min.

FIG. 17 includes two graphs relating to the CO₂R stability performance of the present CO₂R catalyst (CG-medium Cu) in a flow cell at a constant current density of 100 mA cm⁻², in a catholyte being 0.2 M H₂SO₄, with graph (a) showing change in gas product FE, and graph (b) showing change in potential during initial 1 h of CO₂R.

FIG. 18 includes two photographs of the present CO₂R catalyst (CG-medium Cu) before electrolysis (photograph (a)) and after 1 h of electrolysis at 100 mA cm⁻² in 0.2 M H₂SO₄ electrolyte (photograph (c)), and two Cross-sectional SEM images of CG-medium Cu before electrolysis (image (b)), and of CG-medium Cu after 1 h of CO₂R (image (d)).

FIG. 19 include four graphs: graph (a) showing FE towards C₂H₄, graph (b) showing recorded potential at constant current density of 100 mA cm⁻² during 155 h of continuous CO₂R, graph (c) showing single-pass conversion efficiency (SPC) of CO₂ at various flow rate, and graph (d) showing full-cell voltage performance in a slim flow cell at applied current densities from 25 to 125 mA cm⁻², with values are means, and error bars indicate SD (n=3 replicates). The full-cell voltage is the total input voltage to power the CO₂ electrolysis.

FIG. 20 is a schematized exploded perspective view of a slim flow cell according to the system as encompassed herein, with the slim catholyte flow field having a thickness of 0.4 mm.

While the invention will be described in conjunction with example embodiments, it will be understood that it is not intended to limit the scope of the invention to these embodiments. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included as defined by the appended claims.

DETAILED DESCRIPTION

There is provided a CO₂R catalyst and related modified cathode for sustaining electroreduction of carbon dioxide (CO₂) into carbon products in a non-alkali acidic medium. The CO₂R catalyst includes a catalytic layer and an ionic layer comprising a fixed-cationic functional group adsorbed at the surface of the catalytic layer, for improving both a C₂₊ selectivity.

It should be noted that “acid” or “acidic” can qualify a liquid medium, electrolyte, catholyte and/or anolyte having a pH below 7, below or equal to 6, below or equal to 5, below or equal to 4, below or equal to 3, below or equal to 2, or below or equal to 1. A strong acid can be defined as having a pH of at most 3, and a weak acid can be defined as having a pH between 4 and 7. The expression “non-alkali” refers to the absence of alkali cations in solution within the liquid electrolyte. In view of this definition, the non-alkali acidic medium and the non-alkali acidic electrolyte can also be respectively referred as non-alkali cationic acidic medium and non-alkali cationic electrolyte.

The modified cathode includes a gas diffusion layer and the CO₂R catalyst. The modified cathode can be implemented in a CO₂R system for operating electroreduction of gaseous CO₂ into carbon products. The use of the modified cathode in CO₂ electroreduction systems favors the CO₂R reactions (CO₂RR) with respect to the HER by providing a cationic barrier attracting hydroxide ions and maximizing a local alkaline pH at a surface of the cathode. The source of cations is fixed and part of the cathode as an ionic layer comprising cationic functional groups, with the ionic layer being deposited onto the catalytic layer that includes a catalyst sustaining the CO₂RR. The catalytic layer can be referred to as an electrically conducting layer and the ionic layer can be referred to as an ionic conducting layer.

The ionic layer of the CO₂R catalyst provides for the fixation of at least one cationic functional group, for example a benzimidazolium cationic group, within an ionomer. Referring to FIGS. 1 and 2, the ionic layer allows lowering a proton diffusion rate and increasing a local pH to improve C—C coupling at a surface of the catalytic layer (Copper Cu in FIG. 2b). FIG. 2b particularly shows CO₂RR being activated by fixed benzimidazolium cations being adsorbed at the surface of the copper catalyst of the cathode. In contrast to conventional CO₂R in acidic medium where a high current density (˜1 Å cm⁻² as applied by electrochemical potentiostat stations) is necessary to overcome HER, it was found that the modified cathode can achieve satisfactory C₂₊ selectivity at a moderate current density (e.g., ˜100 mA cm⁻²) and thus avoid excessive voltage losses due to cell resistance. It should be noted that satisfactory C₂+ selectivity is at least 40%, at least 50%, at least 60% or at least 70%.

The fixed cationic functional group can be provided as part of an ionomer, i.e., covalently bonded to polymeric backbones of the ionomer. For example, the fixed cationic functional group can from the imidazolium family. For example, the fixed cationic functional group can be as follows.

For example, the cationic functional group can be trimethylammonium, triethylammonium, tributylammonium, tripropylammonium, imidazole, 2-methyl-imidazole, 1,3-dimethyl-imidazole, 1-ethyl-imidazole, 1,3-diethyl-imidazole, 9-carbazole, N-ethyl-carbazole, pyridine, and mixtures thereof.

In some implementations, the modified cathode can further include an additional physical barrier for improving stability of the cathode by preventing the ionic layer to remain in contact with the acidic electrolyte. For example, the physical barrier can be made of an additional carbon-containing layer comprising carbon nanotubes, graphite, or a combination thereof.

Optionally, referring to FIGS. 7a and 7b (FIG. 2a), the carbon-containing layer can be a carbon-Nafion™ layer for providing uniform ionic current distribution and protection from the direct proton flux. For example, as observed in the experimental results, a stable operation (>150 hours) and a 30% increase in total C₂₊ FE of 80% can be thereby achieved.

By the use of the modified cathode as described herein, in a system exempt of alkali metal cations in solution, CO₂R can be achieved in an acidic environment, while associated challenges of salt formation and CO₂ loss are overcome. Fixed-cations within the cationic functional groups of the ionic layer provide buffering of local pH and hydrophobicity that enables CO₂ activation and C—C coupling, even in the presence of a strong acid as catholyte. The local environment can favour C₂₊ production without requiring high current densities that incur severe energy costs.

There is further provided a CO₂ electroreduction system including the modified cathode as described herein. The system can be a membrane electrode assembly (MEA) or a flow cell for operating electroreduction of CO₂ in the non-alkali acidic electrolyte. For example, the flow cell can be a three-electrode flow cell or a slim flow cell using the modified cathode as described herein. Referring to FIG. 20, the system can be a slim flow cell having a cathodic compartment comprising a catholyte inlet being supplied with a non-alkali acidic catholyte, a reactant/cathodic inlet being supplied with gaseous CO₂, the modified cathode as described herein and sustaining reduction of the CO₂ into carbon products, a catholyte outlet to release used catholyte, and a product outlet to release a gas-liquid mixture comprising liquid products and gaseous products. The reactant inlet of the cathodic compartment can be supplied with a CO₂ flow rate between 0.2 and 150 sccm, for example 50 sccm. The catholyte inlet can be supplied with a catholyte flow rate between 1 and 20 mL min⁻¹, for example 5 mL min⁻¹. The system further comprises an anodic compartment and a proton exchange membrane separating the cathodic compartment and the anodic compartment. The anodic compartment can be typical of a flow cell configuration and configured to encase an anode and to be supplied with an acidic anolyte.

In another implementation, the system can be a membrane electrode assembly having a cathodic compartment comprising the modified cathode.

The CO₂ electroreduction system is configured to be supplied with a non-alkali acidic electrolyte. In some implementations, the anolyte can be a solution of sulphuric acid, phosphoric acid, perchloric acid, or any mixtures thereof. In some implementations, when the system is a flow cell, the catholyte can be a solution of sulphuric acid, phosphoric acid, perchloric acid.

The concentration of the anolyte can be between 0.01 and 1 M; preferably from 0.01 to 0.7 M; more preferably from 0.01 and 0.5 M; even more preferably from 0.05 to 0.45 M; most preferably from 0.1 to 0.4 M; and even most preferably from 0.1 to 0.3 M, for example 0.2 M. The concentration can be tailored to a given acidic pH.

The concentration of the catholyte can be between 0.01 and 1 M preferably from 0.01 to 0.7 M; more preferably from 0.01 and 0.5 M; even more preferably from 0.05 to 0.45 M; most preferably from 0.1 to 0.4 M; and even most preferably from 0.1 to 0.3 M, for example, 0.2 M. The concentration can be tailored to a given acidic pH.

There is further provided a process for the electroreduction of carbon dioxide (CO₂) into carbon products in a non-alkali acidic medium including the use of the modified cathode as described herein. In some implementations, the process can include operating CO₂ electroreduction at a moderate current density, i.e., between 10 mA·cm⁻² and 500 mA·cm⁻² as applied by electrochemical potentiostat stations, optionally between 50 and 100 mA·cm⁻². For example, the process can include applying a full-cell potential between 2.75 and 3.4(3.3 V (iR-free)) to operate CO₂R at an industrially relevant reaction rate between 50 and 100 mA·cm⁻². As shown in the experimental results, operation of the system as encompassed herein at a moderate current density in an acidic electrolyte achieved a C₂₊ FE of at least 80% and reduced H₂ FE to at most 10%. For example, the process can include operating at a current density between about 10 mA·cm⁻² and about 500 mA·cm⁻², for example of about 100 mA·cm⁻², to achieve at least between about 70% and about 80% C₂₊ FE and at most between about 10% and 20% H₂ FE in an acid environment, for example in a strong acid environment with pH=0.4, and with no alkali metal cations in solution. Additionally, loss of CO₂ is minimized (e.g. <5%), and the SPC can exceed 70%, 80% or 90%. By avoiding carbonate salt formation from alkali metal ions, the stability of the acidic system is improved and the process can include operating the CO₂R during an effective operation duration of at least 100, 120, 130, 140 or 150 hours without observing a decrease in the C₂₊ FE.

There is further provided a method to manufacture the modified electroreduction cathode as described herein. The method can include providing a gas diffusion layer, depositing an electrically conducting catalytic layer onto the gas diffusion layer and further depositing an ionically conducting layer onto the electrically conducting catalytic layer. The gas diffusion layer can be porous and has a pore size between 0.3 and 1 μm as determined by scanning electron microscope (SEM); preferably from 300 to 900 nm; more preferably from 350 to 800 nm; even more preferably from 400 to 600 nm. For example, the gas diffusion layer has a pore size of 450 nm. Optionally, the gas diffusion layer can be polytetrafluoroethylene (PTFE), hydrophobic carbon paper, or hydrophobic carbon cloth.

Depositing the electrically conducting catalytic layer can include sputtering a metal catalyst onto the gas diffusion layer. Optionally, the sputtering was carried out can be operated in an Angstrom Nexdep sputtering system in a vacuum environment (10⁻⁵˜10⁻⁶ Torr).

The thickness of the electrically conducting catalytic layer can be between 100 nm and 1000 nm as determined by scanning electron microscope cross-section measurements; preferably ranging from 110 to 800 nm; more preferably from 120 to 600 nm; even more preferably from 130 to 500 nm; most preferably from 140 to 400 nm; and even most preferably from 150 to 300 nm; or from 160 to 250 nm.

The method can include controlling a deposition rate of the metal catalyst in accordance with a given thickness of the electrically conducting catalytic layer. For example, the deposition rate can be controlled at 1 Å s⁻¹ to obtain a thickness of the sputtered catalytic layer of 200 nm as determined by scanning electron microscope cross-section measurements when the metal catalyst is pure copper.

For example, the metal catalyst can consist of or comprise copper, nickel, cobalt, tin, bismuth, lead, indium, ruthenium, palladium, zinc, silver, gold, Pd—Cu alloy, Co—Cu alloy, Ni—Cu alloy, Rh—Cu alloy, Ag—Cu alloy, Au—Cu alloy, or any combinations thereof. With preference, the metal catalyst is or comprises copper.

Depositing the ionically conductive layer can include spraying a solution comprising the ionomer being selected for the cationic functional group. The concentration of the ionomer in the solution and the density of the cationic functional group within the ionomer can be tailored to achieve a targeted ion exchange capacity and/or proton permeability of the ionically conductive layer. For example, the ionomer concentration can be between 0.5 wt. % and 5 wt. % based on the total weight of the solution; preferably from 0.5 to 4 wt. %; more preferably from 0.6 to 3 wt. %; even more preferably from 0.7 to 2 wt. % and most preferably from 0.8 to 1.5 wt. %. For example, the ionomer concentration can be 1 wt. %. For example, the cationic functional group density can lead to an ion exchange capacity of the ionomer between 0.5 and 2.6 meq g⁻¹. The method can include controlling a spray loading of the solution in accordance with a given thickness of the ionically conductive layer.

The thickness of the ionically conductive layer can be between 5 μm and 50 μm as determined by scanning electron microscope cross-section measurements; preferably between 12 to 45 μm; more preferably between 15 to 40 μm; and even more preferably from 18 to 30 μm. For example, the spray loading can be maintained at 3 mg cm⁻² with a 1 wt. % ionomer solution to achieve a thickness of 20 μm as determined by scanning electron microscope cross-section measurements.

In some implementations, the method can further include preparing the solution comprising the ionomer. Preparing the solution can include dissolving an ionomer powder in a solvent to yield the desired ionomer concentration in solution. For example, the ionomer can be Aemion™, PiperION, Sustainion®, Fumeion which is selected for the bonded cationic functional groups. For example, the solvent can be a mixture of two solvents, e.g., 80 vol. % ethanol (>99.5%, Sigma-Aldrich)/20 vol. % acetone (>99.5%, Fisher chemical), based on the total volume of solvent. For example, the ionomer includes a cationic function group of the imidazolium family described above, and more particularly benzimidazolium cationic functional groups bonded with methylation poly[2,2′-(2,2″,4,4″,6,6″-hexamethyl-p-terphenyl-3,3″-diyl)-5,5′bibenzimidazole]. The method can include providing the solution with a concentration/density of cationic functional group tailored to maintain a local alkaline pH at a surface of the modified cathode during operation of the electroreduction of CO₂. For example, the concentration/density of cationic functional group tailored to maximize the local pH, and more particularly to obtain the local pH between 11 and 14.

In some implementations, the method can further include depositing the physical barrier layer onto the ionically conductive layer. Depositing the physical barrier layer can include spray coating an ink onto the ionically conductive layer. For example, the ink can include carbon nanoparticle (CNP) provided in an ionomer solution. For example, the ink can include 4 mg CNP (Vulcan XC 72R) and 0.035 g Nafion™ solution (5 wt. %, D520 Dispersion) dispersed in 15 mL methanol (>99.8%, Fisher chemical). Optionally, the ink can be sonicated for a given period prior to spray coating to keep the CNP uniformly dispersed.

It should be noted that the modified cathode can be referred to as a Cationic Group (CG)-modified electrode, CG-modified cathode, or CG-modified Cu electrode when the catalyst is copper. The combination of the ionic layer and catalytic layer can be referred to as a CG-modified catalyst being part of the cathode that is used in the electroreduction systems described herein. An ionomer loading refers to a weight of ionomer that is sprayed onto the surface of the catalyst (per cm² of such surface), thereby including a weight of the polymer backbone. (CG)-low, -medium and -high loading refers herein to a density of the cationic functional group in the ionic layer that consists of or comprises the ionomer. Ion exchange capacity can be used to denote the cationic functional group density as low, medium or high loading in the ionic layer. Depending on the loading of the cationic functional group(s) in the ionic layer, the (CG)-modified electrode can be referred to as

• • a CG-low electrode having a cationic functional group with the ion exchange capacity between 0.5 and 1.4 meq·g⁻¹; preferably between 0.5 and 1.2 meq·g⁻¹; more preferably between 0.5 and 1.0 meq·g⁻¹; even more preferably between 0.5 and 0.8 meq·g⁻¹; and most preferably between 0.5 and 0.6 meq g⁻¹, for example, 0.5 meq g⁻¹;
  • a CG-medium electrode having a cationic functional group with the ion exchange capacity between 1.0 and 2.3 meq·g⁻¹; preferably between 1.1 and 2.1 meq·g⁻¹; more preferably between 1.2 and 2.0 meq·g⁻¹; even more preferably between 1.3 and 1.9 meq·g⁻¹; and most preferably between 1.4 and 1.7 meq g⁻¹, for example, 1.5 meq g⁻¹; and
  • a CG-high electrode having a cationic functional group with the ion exchange capacity between 1.8 and 2.6 meq·g⁻¹; preferably between 2.0 and 2.6 meq·g⁻¹; more preferably between 2.1 and 2.6 meq·g⁻¹; even more preferably between 2.3 and 2.6 meq g⁻¹, for example, 2.5 meq g⁻¹.

In some implementations, electroreducing CO₂ into carbon products can include tailoring a cationic functional group loading in the ionic layer to maximize the C₂₊ Faradaic Efficiency (FE), for example to achieve at least 60% C₂₊ FE, at least 70% C₂₊ FE, or at least 80% C₂₊ FE—and to minimize the H₂ FE to at most 20%, at most 15% or at most 10%.

EXPERIMENTAL RESULTS

Studies on the Alkali Cation Effect on Promoting CO₂R.

CO₂R in acidic media struggles from low Fes due to the overabundance of H₃O⁺ near the catalyst surface. To activate CO₂R in strongly acidic environments, the local alkalinity and the presence of alkali cation must be increased. In pure H₂SO₄ electrolyte without any alkali metal cation, only traces of CO₂R products at low concentration H₂SO₄ (0.01 M) were detected. The potential is close to −10V vs. due to the large system and charge transfer resistance. The CO₂R products disappeared quickly when the concentration was raised to above 0.05 M FIG. 3). Improved selectivities toward C₂₊ products were observed in acidic electrolyte with increasing alkali cation concentration (FIG. 4). However, solid salt formation on the catalyst and the gas diffusion electrode is typically observed after a few hours of operation that causes flooding and hence the loss of CO₂R FE (FIG. 5).

Alkali Cation Vs. Fixed-Cation Functional Group in Acidic Media

Alkali cations facilitate CO₂R in strongly acidic environment. In addition to the extensive studies on the role of alkali cations in promoting CO₂R, prior work has suggested that fixed-cations in ionic liquids or surfactants—including ammonium cations, imidazolium cations, and benzimidazole cations—can also stabilize key intermediates and promote CO₂R to multi-carbon products. Using density functional theory (DFT), the fixed-cation effect on CO₂ adsorption/activation and C—C coupling was observed by comparing the adsorption Gibbs free energy of CO₂ on Cu(100) (ΔG_*CO2) with an alkali cation (K⁺) and a fixed-cation functional group (benzimidazolium). It was found that K⁺ and the fixed-cation functional group at the Cu(100) surface induce CO₂ adsorption with similar ΔG_*CO2. In both cases, CO₂ adsorbs on the Cu(100) surface in a η²_C,O conformation (FIGS. 2b and 2c). The energy barriers for C—C coupling were also similar, with 0.61 and 0.58 eV in the presence of K⁺ and fixed-cation functional group, respectively. These simulations indicate the potential to replace the function of an alkali cation in acidic CO₂R with that of a fixed-cation functional group.

Preparation of CG-Modified Cu Electrodes

The Cu electrode was prepared by sputtering pure Cu (>99.99%, Kurt J. Lesker) onto a polytetrafluoroethylene (PTFE) gas diffusion layer with 450 nm pore size. The sputtering was carried out in an Angstrom Nexdep sputtering system in a vacuum environment (10⁻⁵˜10⁻⁶ Torr) with a deposition rate of 1 Å s⁻¹. The thickness of the sputtered Cu layer was 200 nm as determined by scanning electron microscope cross-section measurements.

The CG-modified Cu electrodes were fabricated by spraying 1 wt. % ionomer solutions with different cationic functional group densities and the spray loadings were kept constant at 3 mg cm⁻².

The ionomer was prepared by dissolving ionomer powder (Aemion™ from Ionomr) in a solvent composed of 80/20 vol. % ethanol (>99.5%, Sigma-Aldrich)/acetone (>99.5%, Fisher chemical) to yield 1 wt. % ionomer solution. Carbon protected CG-Cu was prepared by spray coating carbon nanoparticle (CNP) ink onto the CG-modified Cu. The CNP ink was composed of 4 mg CNP (Vulcan XC 72R) and 0.035 g Nafion™ solution (5 wt. %, D520 Dispersion) dispersed in 15 mL methanol (>99.8%, Fisher chemical). The CNP ink was sonicated for at least 1 hour prior to spray coating.

Electrochemical Reduction of CO₂

The CO₂R was carried out in a three-electrode flow cell, where CG-modified Cu as the cathode electrode with an exposed size of 1 cm², an Ag/AgCl (3 M KCl) as the reference electrode, and a platinum gauze (99.99%, Sigma-Aldrich) as the counter electrode (anode). The H₂SO₄ catholyte and anolyte (>95%, ACS reagent, Sigma-Aldrich), with concentrations varied from 0.01 M to 0.2 M, were circulated in the flow cell at a constant flow rate of 10 mL min⁻¹. The catholyte and anolyte were separated by a proton exchange membrane (Nafion™ 117). The CO₂ was supplied at a flow rate of 50 sccm by using a digital mass flow controller. All the electrochemical tests were performed though a potentiostat (Autolab PGSTAT302N). The volumes of catholyte and anolyte used for circulation were 25 mL, and the liquid products were collected after 1 hour of continuous operation for analysis. Linear sweeping voltammetry (LSV) measurements were carried out in the same flow cell at a scan rate of 50 mV s⁻¹. All potentials were converted to reversible hydrogen electrode (RHE) via the following equation:

E ⁢ ( RHE ) = E ⁢ ( Ag / AgCl + 0.21 V + 0.059 × pH ( 1 )

The ohmic resistance and charge transfer resistance were measured through electrochemical impedance spectroscopy (EIS), and the data points were obtained between 0.01 Hz and 200 kHz.

CO₂R Product Analysis

The gas products were analyzed through gas chromatograph (Perkin Elmer Clarus 590) equipped with a thermal conductivity detector (TCD) and flame ionization detector (FID). The gas products were controlled in 1 mL volume from the gas outlet and injected into the gas chromatograph for quantification, and the Faradaic efficiency was calculated via the following equation:

Faradaic ⁢ efficiency ⁢ ( % ) = z ⁢ F ⁢ P R ⁢ T × v × 1 I × 1 ⁢ 0 ⁢ 0 ⁢ % ( 2 )

where z represents the number of electrons required to produce the product, F represents the Faraday constant, P represents the atmosphere pressure, R represents the ideal gas constant, T represents the temperature, v represents the gas flow rate at the gas, and I represents the total current.

The liquid products were analyzed using proton nuclear magnetic resonance spectroscopy (¹H NMR, 600 MHz Agilent DD2 NMR Spectrometer) under water suppression mode. Dimethyl sulfoxide (DMSO) was used as the reference and deuterium oxide (D20) as the lock solvent. The Faradaic efficiency of liquid was calculated via the following equation:

Faradaic ⁢ efficiency ⁢ ( % ) = znF It × 1 ⁢ 0 ⁢ 0 ⁢ % ( 3 )

where z represents the number of electrons required to produce the product, n represents the mole number of products, F represents the Faraday constant, I represents the total current, and t represents the electrolysis time.

The single-pass CO₂ conversion efficiency (SPC) of CO₂ was calculated using the following equation:¹³

SPC ⁢ ( % ) = j z ⁢ F × V m flow ⁢ rate × 1 ⁢ 0 ⁢ 0 ⁢ % ( 4 )

where j represents the partial current density of a specific product, z represents the number of electrons required for the specific product, F represents the Faraday constant, V_m represents the molar volume.

Materials Characterization

Scanning electron microscopy (SEM) cross-section images were obtained in a high-resolution scanning electron microscope (HR-SEM, Hitachi S-5200). In situ Raman measurements were obtained using a Renishaw inVia Raman microscope equipped with a water immersion objective (63×) with a 785-nm laser in a modified flow cell. Considering that at current densities greater than 100 mA cm⁻² as applied by electrochemical potentiostat stations, H₂ bubbles generated from Cu will cover the lens and deteriorate the quality of Raman signals, 5 mM H₂SO₄ was used for all the tests and applied a constant current density of 20 mA cm⁻² for all the in situ measurements. For the potassium-rich cases, 3 M KCl was added in the 5 mM H₂SO₄ electrolyte. CO₂ was supplied to the cathode during all the in situ measurements.

Slim Flow Cell Configuration

The cathodes for the slim flow cell were the CG-medium Cu electrodes with a CG-medium ionomer loading of 3 mg cm⁻². The anodes were made by IrO₂ coated Ti felt (0.3 mm thickness) with a loading of 1 mg cm⁻². The measurements were performed in a slim flow cell with an active area of 1 cm⁻² accessed with a serpentine channel. The catholyte was circulated in a cathodic flow field with a thickness of 0.4 mm as shown in FIG. 20. CO₂ was fed into the cathode at a flow rate of 50 sccm by using an accurate mass flow controller. The 0.2 M H₂SO₄ catholyte and anolyte were circulated at a constant flow rate of 5 ml min⁻¹ using peristaltic pumps. A cation exchange membrane (Nafion™ 117, Fuel Cell Store) was used for ion exchange and separation of the cathode and anode.

DFT Calculations

Spin-polarized DFT calculations were performed with the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional as implemented in the Vienna Ab-initio Simulation Package (VASP). Plane-wave cutoff energy was set to 450 eV and a F-centered k-point sampling of 2×3×1 which generated by the Monkhorst-Pack scheme were used. The zero damping DFT-D3 method of Grimme was taken into consideration to better describe the long-range van der Waals interactions. A monolayer of water molecules was included in models to explicitly account for the solvation effect when calculating the energies of CO₂ adsorption and C—C coupling. A solvated potassium and benzimidazolium cation were placed on the top of CO₂ and C—C coupling (*OCCO) intermediates and water molecules, in a 6×4×3 periodic cell exposing the (100) facet of fcc Cu with a vacuum layer of 20 Å in thickness.

Tuning the CG Layer to Improve C₂₊ Productivity in Non-Alkali Electrolyte

Inspired by the DFT calculations, a fixed-cation enrichment strategy was pursued by deploying a layer of ionomer with the fixed-cationic functional group, one that could vary the concentration of positive benzimidazolium groups on the Cu catalyst. The number of the fixed-cation groups per dry polymer is indicated by its ion exchange capacity (IEC). A low concentration of fixed-cation groups (CG-low) that has the lowest water uptake was firstly introduced, which was expected to maximize the local pH and minimize proton migration to the Cu catalyst.

CG-low, medium, high loadings denote a concentration/density of cationic functional groups per mass of ionomer where low is least concentrated and high is the most concentrated, whereas the ionomer loading is to be understood as the mass of ionomer per unit area. To achieve efficiently high C2+ FE, both density of the cationic functional groups and loading of the ionomer can be controlled. However, increasing the ionomer loading does not necessary affect the concentration of cation group per mass of ionomer. The CO₂R performance of CG-low Cu catalyst in a flow cell with a gas diffusion electrode was first examined, employing 0.2 M H₂SO₄ as both catholyte and anolyte. Varying the CG-low loading did not affect the performance when there is full coverage of the fixed-cation group layer (FIG. 6). The CG-low Cu showed an FE towards H₂ of 60%, and CO₂R products were observed at current densities of 50 and 100 mA cm⁻² (FIG. 7a)—an improvement over the negligible amount of CO₂R (<1%) observed on the bare Cu case at similar electrolyte bulk pH with K⁺. To elucidate the reason for this improvement, cyclic voltammetry (CV) of CG-low Cu and bare Cu in H₂SO₄ electrolytes was carried out, with and without K⁺. The onset of the plateau that represents the depletion of H₃O⁺ at the Cu surface is at a lower current with CG-low Cu compared to bare Cu in K⁺ rich electrolyte (FIG. 7b), indicating a proton barrier effect and higher local pH in the CG-low case. We posited that CO₂R activity could be further enhanced with a higher fixed-cation group population that aimed to further stabilize negatively charged CO₂ intermediates.

To test this hypothesis, control studies to examine the performance of CG-low Cu in H₂SO₄ with the addition of KCl were carried out. Increased CO₂R FE was detected, especially towards C₂₊ products such as C₂H₄(FE 27±2%) at 100 mA cm⁻² with 3 M KCl (see FIGS. 7c and 8). This result indicated that the presence of alkali cation accelerates the CO₂R kinetics on the CG-low Cu surface. However, the excess of local OH⁻ and K⁺ ions lead to salt formation at the catalyst and gas channel layers, and a rapid decay in C₂H₄ FE from 27% to 15% with an increase in H₂ FE from 20% to 48% (FIG. 7d).

To explain the initially lower HER and improvement in C₂₊ FE, the CV and electrical double layer (EDL) capacitance for the electrolyte was measured, with and without K⁺ on CG-low Cu. With the additional cations, the onset of the current plateau of H₃O⁺ depletion takes place at slightly lower current and overpotential (FIG. 9). The additional cation thus further confines the EDL, and decreases the rate of H₃O⁺ migration towards the outer Helmholtz plane (OHP). In the absence of the K⁺, the benzimidazolium group is the only cation that can form the EDL at the electrode/electrolyte interface. With the addition of KCl in the electrolyte, an increase in the double layer capacitance was observed, indicating a more compact cation layer with the binding of hydrated K⁺ near the Cu surface (FIG. 10). In situ surface enhanced Raman spectroscopy (SERS) measurements on CG-low Cu in both electrolytes (with and without KCl) was performed to qualitatively assess the cation layer effect on CO adsorption, the main intermediate in CO₂R (FIG. 7e). An obvious blueshift of Cu—CO vibrational frequency from 350 cm⁻¹ to 370 cm⁻¹ was observed when K⁺ was added to the H₂SO₄ solution, indicating a stronger binding of CO on Cu surface—a prerequisite for carbon-carbon coupling. The improvement in the C₂₊ selectivity is thus attributed to the increase in K⁺ concentration in solution at OHP, forming a more compact EDL that suppresses HER and enhances C—C coupling, albeit at the cost of salt formation and catalyst degradation.

To further improve the FE towards C₂₊ products without relying additional alkali cations in solution, the fixed-cation group concentration was increased. The performance of higher fixed-cation group concentration loaded samples was evaluated, CG-medium and CG-high modified Cu, at the same condition (pure 0.2 M H₂SO₄). The increased fixed-cation group concentration was confirmed via IEC (Table 1).

TABLE 1
Metrics of ionomers with benzimidazolium
cationic functional groups.

		IEC*	Conductivity	Water uptake**
	Type	(meq g−1)	(mS cm−1)	(%)
	CG-low	0.5-0.6	1.3-2.5	13-15
	CG-medium	1.4-1.7	2-4	35-50
	CG-high	2.3-2.6	8-11	95-100
	*IEC in the hydroxide (OH−) counter-ion form.
	**Approximate swelling properties when cast into membrane form at 25-50 μm.

The higher fixed-cation group concentrations were further confirmed through SERS and surface capacitance. The solid-state spectrum of the CG-medium and CG-high showed higher peak intensity from 900 cm⁻¹ to 1100 cm⁻¹, the frequencies that are associated with the breathing vibration of benzene rings in the benzimidazolium unit (FIG. 11a). There is an observed increase of double layer capacitance for CG-medium and CG-high (FIG. 11b). Similar blueshifts of Cu—CO vibrational, as when K⁺ was added to the electrolyte, were observed when the concentration of fixed-cation group is increased, indicating a stronger binding of CO on Cu surface (FIG. 11c).

Despite the merits of a high cation group concentration and high ionic conductivity, high IEC will also lead to less hydrophobicity of the fixed-cation group layer and greater proton access. In acidic electrolyte, this results in rapid proton influx and a lower local pH unfavorable for CO₂R. To test this hypothesis, the CV curve of three different concentration fixed-cation group samples (CG-low, medium and high) was measured in pure H₂SO₄ electrolyte (0.2 M) to investigate the proton depletion current. As expected, the plateau current of HER increased with the increasing IEC and water uptake, indicating a faster H₃O⁺ migration from the bulk electrolyte when a higher fixed-cation group concentration layer is applied (FIG. 11d). Although a higher fixed-cation concentration is desired at OHP, the loss of the hydrophobicity compromises the effectiveness of the CG-layer as proton-barrier. It is therefore important to tune both the functional group density and water/proton permeability to reach the highest C₂₊ FE. The water permeability can be varied by the cationic functional group density. Since the cationic functional groups are highly hydrophilic, introducing more functional groups will also result higher water/proton uptake and permeability.

The performance of CG-modified Cu at a range of acid concentrations (FIG. 12) was then tested. The best result sustained over a one-hour test was that of CG-medium Cu, with an FE of 40±2% towards C₂H₄, much lower HER (<10%), and a total C₂₊ FE of 80±3% at current density of 100 mA cm⁻² (FIGS. 11e and 11f). CG-high resulted in more CO detected (28±2%) and less total C₂₊ products to 60±4%. This poorer performance was attributed to the higher water uptake and associated proton flux in the case of CG-high, negating the benefit of increased cation group concentration.

To confirm the role of surface cation concentration, CG-medium and CG-high Cu was tested with the addition of 3 M KCl. In contrast to the substantial improvement obtained with CG-low Cu, the additional cations decreased C₂H₄ FE to ˜25% and increased H₂ to 51% for CG-medium Cu. Based on in situ SERS and CV measurements, it was expected that cation groups were replaced with K⁺ at OHP, increasing water permeability and HER (FIGS. 13 and 14). Similar trends were also observed for CG-high Cu with poorer performance (20% C₂H₄ and 72% H₂) following the addition of cations in solution (FIG. 15).

Double layer capacitance of CG-medium and high modified Cu was measured, which slightly increased with the addition of KCl in the electrolyte, but to an extent much less than that observed on CG-low Cu, suggesting that instead of forming a more compact layer with K⁺, the hydrated cation group is more likely to be replaced by the K⁺ at OHP. To further probe the electrode and electrolyte interface, in situ SERS was conducted and it was found that with the addition of K⁺, the characteristic vibration signal of benzimidazolium group from 900 cm⁻¹ to 1100 cm⁻¹ actually decreased on CG-medium and CG-high modified Cu, suggesting that the benzimidazolium cation groups were no longer the dominant species and were likely replaced by the hydrated K⁺ cations that migrate readily to the surface under the negative biased potential due to the higher water uptake in CG-medium and CG-high (FIG. 13).

Taken together, the OHP formed on CG-medium (and CG-high) modified Cu in high potassium salt electrolyte is mainly composed of K⁺ instead of benzimidazolium cation group, thus leading to performance close to that of bare Cu in 0.2 M H₂SO₄ and 3 M KCl electrolyte. Replacing the surface cation group with K⁺ would likely accelerate the electron transfer for HER based on CV measurements (FIG. 14), leading to a significant increase in the H₂ FE.

This is thus further confirmed by the lower C₂₊ selectivity on CG-medium after adding more K⁺ sources into the electrolyte. Similar to the CG-low case, the additional K⁺ containing electrolytes resulted in severe salt precipitation and rapid performance degradation (FIG. 16). In non-alkali acidic electrolyte, CG-medium creates a productive catalytic condition that balances cation group density and water uptake. Concentrated fixed non-alkali cation groups enable stable CO₂R performance with H₂ FE less than 10% and C₂₊ FE around 80%.

Acidic CO₂R Performance with the CG Modified Catalyst

Replacing the alkali metal cation in the solution with a layer of fixed-cation groups on Cu should reduce salt formation and thereby remove a leading cause of poor system stability. However, CG-medium Cu (in 0.2 M H₂SO₄) provided C₂H₄ FE>35% for less than 1 hour (FIG. 17). Dark spots formed on the Cu (FIG. 18) were observed, which were attributed to the corrosion of Cu as the protective cation group layer decayed over time, followed by an uneven distribution of current density. The hydration of cation groups over the layer proceeds at different rates, leading to nonuniformities in hydration, ionic conductivity, and access for protons to oxidize the Cu underneath. A layer of carbon nanoparticles was applied atop the CG layer to serve as a physical barrier to limit the proton transfer. This layer was made up of carbon nanoparticles (CNP) bound with Nafion™ to prevent breakdown of the CG layer due to nonuniform hydration and ionic conductivity. The Nafion™/carbon-protected CG-medium Cu operated for over 150 hours in strong acid, with a steady C₂H₄ FE of over 40% (FIGS. 19a and 19b). Only trace K+ was detected in the electrolyte after 80 hours of electrolysis (table 2). This marks an improvement of over an order of magnitude compared to the best prior acidic CO₂R.

TABLE 2
Quantification of alkali cations in the electrolyte*
by inductively coupled plasma (ICP) measurements

	Li+	Na+	K+**	Cs+

	Concentration (ppm)	<0.001	0.178	2.514	<0.001
	*The electrolyte was taken after 80 h electrolysis
	**The trace K+ comes from the Ag/AgCl electrode which is filled with 3M KCl

The lack of salt formation in the proposed systems also benefits CO₂ utilization.

CO₂ is regenerated within the bulk electrolyte. In some implementations, the regenerated CO₂ can be fed back to the gas inlet. A high SPC was pursued to reduce further the product separation costs. SPC is calculated using the fraction of the input CO₂ supply that is converted to CO₂R products. By throttling the input CO₂, 90% SPC for all CO₂R products (FIG. 19c) was achieved. Further employing a slim flow cell with low resistance (FIG. 20), the carbon coated CG-medium Cu was operated at current densities from 50 to 150 mA cm⁻², and the full-cell voltage at 100 mA cm⁻² was 3.3 V (without iR compensation) with a C₂₊ FE of 80%, leading to a record energy efficiency (EE) of 28% for acid CO₂R system (FIG. 19d).

It is worth mentioning that throughout the following description when the article “a” is used to introduce an element it does not have the meaning of “only one” it rather means of “one or more”. For instance, the unit according to the invention can be provided with one or more reaction and/or separation chamber, one or more confining openwork structure, etc. without departing from the scope of the present invention. It is to be understood that where the specification states that a component, feature, structure, or characteristic “may”, “might”, “can” or “could” be included, that particular component, feature, structure, or characteristic is not required to be included.

Although the embodiments of the systems and corresponding parts thereof consist of certain geometrical configurations as explained and illustrated herein, not all of these components and geometries are essential and thus should not be taken in their restrictive sense. It is to be understood, as also apparent to a person skilled in the art, that other suitable components and cooperation thereinbetween, as well as other suitable geometrical configurations, may be used for the systems as encompassed herein, as can be easily inferred herefrom by a person skilled in the art. Moreover, it will be appreciated that positional descriptions and illustrations should, unless otherwise indicated, be taken in the context of the figures and should not be considered limiting.

In the following description, the term “about” means within an acceptable error range for the particular value as determined by one of ordinary skill in the art, which will depend in part on how the value is measured or determined, i.e., the limitations of the measurement system. It is commonly accepted that a 10% precision measure is acceptable and encompasses the term “about”.

It should be understood that any one of the above-mentioned aspects/embodiments/implementations of each of the modified cathode, process, method, system and use of a modified cathode may be combined with any other of the aspects thereof, unless two aspects clearly cannot be combined due to their mutually exclusivity. In the above description, an embodiment or implementation is an example of the invention. The various appearances of “one embodiment,” “an embodiment”, “some embodiments”, or “some implementations” do not necessarily all refer to the same embodiments. Although various features of the invention may be described in the context of a single embodiment or implementation, the features may also be provided separately or in any suitable combination. Conversely, although the invention may be described herein in the context of separate embodiments or implementations for clarity, the invention may also be implemented in a single embodiment or implementation.

All publications detailed herein are incorporated by reference.

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